In a bowl, beat the yoghurt with the sugar, add the baking soda, oil, cocoa and sifted flour.
Pour the thick mixture into a tray smeared with a little oil and sprinkled with flour, bake in a preheated 356°F (180 °C) oven about 20 min.
To make the topping, put all of the ingredients for it in a metal bowl and put to boil in a water bath to form a smooth, homogeneous mixture.
Poke holes in the layer in several places with a toothpick, cut it into the desired slices (squares, rhombuses, rectangles) and pour the topping with a spoon over it evenly.
Sprinkle with the walnuts on top and leave to cool.
An amazing brownie with an amazing taste!